1. Field of the Invention
The present invention relates to a high temperature superconductor that has low superconducting anisotropy, a high critical current density (Jc), a high irreversible field (Hirr) and a long coherence length "xgr"c in a direction perpendicular to the plane (the direction being the c-axis direction, the intrafacial plane the ab-axis), and a superconducting transition temperature (Tc) of, for example, 100 K or more, and to a method for producing the superconductor.
2. Description of the Prior Art
A high Tc has been considered to be closely related to high superconducting anisotropy (two-dimensionality) of superconducting properties. Known high-temperature superconductors having a two-dimensional layered structure comprised of charge reservoir layers and superconducting layers include Y, Bi, Tl, and Hg based copper oxide superconductors.
Moreover, because in these superconductors having a layered structure the charge reservoir layer is an insulation layer or non-superconducting layer with low superconductive coupling in the c-axis direction and, therefore, a small interactive effect between superconducting layers, in addition to which the coherence length along the c-axis is short, superconducting anisotropy xcex3 is large, being in the order of 4-5 to 300 (xcex3 defined as the ratio of the coherence length, the square root of the electron effective mass ratio, or magnetic field penetration depth ratio, is xcex3="xgr"ab/"xgr"c=(mc/mab)xc2xd=xcexc/xcexab).
As such, the Jc, especially the Jc under a high magnetic field, and the Hirr, the upper limit magnetic field at that electrical resistance generates, become small, posing many problems to the practical use as wire or bulk superconducting material.
Furthermore, the large superconducting anisotropy means that the coherence length "xgr"c in the c-axis direction is small, so that when used as a superconducting device material, the properties of the layered-structure-typed superconducting device are not adequate, especially the Josephson current density. In JP-A-Hei 8-183614, the inventors including the present inventor proposed a high-temperature superconductor having low superconducting anisotropy, a long coherence length in the c-axis direction and a large current density under a high magnetic field, using a matrix of copper, oxygen and alkaline earth metal elements instead of Tl, Hg and other such elements that are harmful and scarce.
There are reports claiming low superconducting anisotropy with respect to Hg-based Hg-1201 (HgBa2CuO4+y) and infinite layer structure superconductors (Sr1xe2x88x92xLaxCuO2), but based on research by the present inventor, such superconductors are not a low anisotropic superconductor.
An object of the present invention is to provide a high-temperature superconductor that is low in superconducting anisotropy, with superconducting electrons being able to move not only in the plane of the layers (ab-axis direction) but also perpendicular thereto, and exhibits a practicably large critical current density under high temperature and high magnetic field conditions, and a method for producing the superconductor.
Another object of the present invention is to provide a high-temperature superconductor that is low in superconducting anisotropy and excellent in magnetic properties which provides strong coupling between the superconducting layers mediated by the charge reservoir layer, high magnetic-flux pinning capability and high zero resistance-magnetic field, and a method for producing the superconductor.
Still another object of the present invention is to provide a low superconducting anisotropy, high-temperature superconductor having a long coherence length in the c-axis direction, and a method for producing the superconductor.
In accordance with the present invention, the above object is attained by providing a high-temperature superconductor having low superconducting anisotropy, comprising a two-dimensional layered structure of lattice unit cells each constituted by a pair of superconducting layer and charge reservoir layer provided perpendicularly on the superconducting layer, wherein at least a portion of atoms comprising the charge reservoir layer are replaced by atoms giving superconductivity, metallizing or rendering the charge reservoir layer superconducting, a thickness of the superconducting layer is increased, and therefore coherence length in a thickness direction is increased based on the uncertainty principle, lowering superconducting anisotropy.
Copper and oxygen are examples of atoms giving superconductivity. The low superconducting anisotropy, high-temperature superconductor may be produced by a method comprising supplying the superconductor starting material onto a single-crystal substrate or crystal-oriented substrate, sealing the substrate in an oxidation resistant capsule and applying a prescribed pressure and heat to the capsule.
Thus, as described in the foregoing, by increasing the superconducting layer thickness of the superconductor according to this invention, the superconducting electron uncertainty region (thickness) in the perpendicular direction (c-axis direction) is expanded according to the uncertainty principle, making it possible to increase the coherence length in the direction of the c-axis, thereby enabling the superconducting anisotropy to be reduced to a very low level.
Moreover, by replacing some or all of the atoms constituting the charge reservoir layer existing alternately with the superconducting layer with atoms giving superconductivity, thereby metallizing or rendering the charge reservoir layers superconducting, strengthening the superconducting coupling and allowing free movement of the superconducting electrons.
The above and other features of the present invention will become apparent from the following description made with reference to the drawings.